(1.) Field of the Invention
The present invention relates to a phase modulation method and apparatus used in a digital communication system.
(2.) Description of the Prior Art
Multiple phase shift keying (MPSK) is a well known modulation system in which there are as many phase positions of a carrier wave as there are digital information input code elements. For example, with a quaternary code, there are four different phase positions, i.e., 0.degree., 90.degree., 180.degree., and 270.degree. of the carrier wave. In such a modulation system, it is required that the phase positions of the modulated waves be exactly equal to the desired phase positions and that the amplitude of the modulated waves in the different phase positions be maintained at a constant value.
Since components in a phase modulator have different characteristics, if the phase positions of the modulated waves deviate from the desired phase positions, for example, in the case of a quadrature phase shift keying (QPSK) system, the phase differences of the modulated waves between neighboring phase positions would not be equal to 90.degree.. Furthermore, because of the differences in characteristics of the components, the amplitude of the modulated waves in the different phase positions cannot be equally controlled.